1. Field of the Invention
The present invention relates to a photo-detecting apparatus for detecting only a signal light component by removing background light components from light input to a photodiode.
2. Related Background Art
A photo-detecting apparatus comprises one or more photodiodes, the electric signal current from each photodiode being integrated by an integrating circuit, and a signal voltage formed by the result of this integration being output. Furthermore, depending on the photo-detecting apparatus, the aforementioned signal voltage, which is an analogue signal, may be converted to a digital signal A/D conversion), and this digital signal is then output. If the signal voltage exceeds a prescribed value in this A/D conversion process, then the digital signal output by A/D conversion on the basis of this signal voltage will become saturated at a value corresponding to this prescribed value, and consequently, it will not be possible to perform accurate light detection. Therefore, in the prior art, saturation such as that described above has been prevented from occurring by setting the aforementioned prescribed value to the maximum predictable value of the aforementioned signal voltage, or a value thereabove. Furthermore, in some cases, the dynamic range is expanded by using a technique such as logarithmic compression, or the like.
Photo-detecting apparatuses are for example used in a range sensor incorporated into cameras. In a range sensor of this kind, the reflection of spot light beams projected onto an object by light projection means, such as a light-emitting diode, or the like, is detected respectively by two photo detecting apparatuses, and the distance is measured on the basis of the two detected signal outputs. In this case, since the intensity of the background light is also superimposed and detected when detecting the intensity of the spot light (signal light), then the accuracy of distance measurement can be improved by detecting only the intensity of the background light by means of the two respective photo-detecting apparatuses, whilst no spot light is being projected, and then detecting only the intensity of the spot light by taking the differential readings in either detector
However, in an integrating circuit used in a conventional photo-detecting apparatus, since no measures are taken with respect to noise components which have different values for each integrating operation, such as thermal noise generated by amplifiers, or the like, a noise error may arise. Therefore, if the light intensity detected by the photodiode, in other words, the value of the aforementioned signal voltage, is smaller than the noise component which varies for each integrating operation, then the S/N ratio in the optical detection will be low.
Furthermore, in A/D conversion in a conventional photo-detecting apparatus, the aforementioned prescribed value is set to a large value in order to prevent the signal level from becoming saturated, but if the light intensity detected by the photodiode, in other words, the value of the aforementioned signal voltage, is small, then the resolution of the digital signal output will be decreased.
Moreover, problems of the following kind arise if the intensity of the spot light is selectively detected by subtracting the detection results for the intensity of the background light from the detection results for intensity of the spot light and intensity of the background light, as in the case of a photo-detecting apparatus used as a range sensor.
Specifically, if the intensity of the background light is large compared to the intensity of the spot light, then the aforementioned signal voltage when the intensity of the spot light is detected with the intensity of the background light superimposed thereon will be very large, and hence, in order to prevent saturation of the signal occurring in this case, it is necessary to set the aforementioned prescribed value to an ever greater value. This causes further degradation of the resolution of the digital signal output on the basis of the intensity of the spot light obtained by the subtraction operation.
As described above, in a conventional photo-detecting apparatus, the S/N ratio is poor and the resolution of the digital signal output when A/D conversion is performed, is also poor. Therefore, the present invention was devised with the foregoing in view, an object thereof being to provide a photo-detecting apparatus having an excellent S/N ratio. Moreover, it is a further object of the invention to provide a photo-detecting apparatus which has excellent resolution when performing A/D conversion, even if the light intensity is small, without the signal becoming saturated when the light intensity is large.
The photo-detecting apparatus relating to the present invention comprises, (1) a photodiode for outputting an electric signal current corresponding to the intensity of the incident light; (2) an integrating circuit for integrating electric charge according to the electric signal current output by the photodiode and outputting a signal voltage corresponding to the electric charge thus integrated; (3) a CDS (Correlated Double Sampling) circuit having: a first capacitor and amplifier provided in sequence between an input terminal inputting the signal voltage output by the integrating circuit and an output terminal, a second capacitor and third capacitor having mutually equal capacitance values, provided in parallel between the input and output of the amplifier, and switching means for selecting either one of the second or third capacitors so as to integrate electric charge corresponding to the amount of change in the signal voltage; and (4) a differential calculating circuit for determining the differential between the electric charges integrated respectively in the second and the third capacitors of the CDS circuit, and outputting a signal voltage corresponding to this differential.
According to this photo-detecting apparatus, the electric signal current corresponding to the intensity of the incident light is output by the photodiode, and in the integrating circuit, electric charge is integrated according to the electric signal current output by the photodiode, and a signal voltage is output in accordance with the amount of electric charge thus integrated. In the CDS circuit, the electric charge signal output by the integrating circuit is input to the first capacitor, and an electric charge corresponding to the change in the signal voltage thus input is integrated in either the second or third capacitor as selected by the switches. Thereupon, in the differential calculating circuit, the differential in the electric charges integrated respectively by the second and third capacitors of the CDS circuits is determined and a signal voltage corresponding to this differential is output.
Furthermore, the photo-detecting apparatus according to the present invention is a photo-detecting apparatus further comprising a timing control circuit for controlling the respective operations of the integrating circuit, the CDS circuit and the differential calculating circuit, and the photo-detecting apparatus is used in conjunction with light projecting means for projecting spot light onto an object; wherein the timing control circuit causes: (1) a first electric charge to be integrated in the second capacitor of the CDS circuit on the basis of the amount of change of the signal voltage output by the integrating circuit, when the photodiode detects (receives) a spot light component and a background light component, in a first period wherein the spot light is projected onto the object by the light-emitting means; (2) a second electric charge to be integrated in the third capacitor of the CDS circuit on the basis of the amount of change in the signal voltage output by the integrating circuit when the photodiode detects (receives) the background light component, in a second period wherein the spot light is not projected onto the object by the light-emitting means; and (3) the differential between the electric charges integrated respectively by the second and the third capacitors of the CDS circuit to be calculated by the differential calculating circuit, in a third period after the first and the second periods, the signal voltage corresponding to this differential being output by the differential calculating circuit.
In this case, under the control of the timing control circuit, in the first period, a first electric charge corresponding to the change in the signal voltage output by the integrating circuit when the spot light component and background light component are detected by the photodiode is integrated in the second capacitor of the CDS circuit. Moreover, in the second period, a second electric charge corresponding to the change in the signal voltage output by the integrating circuit when the photodiode detects the background light component is integrated by the third capacitor of the CDS circuit. And during the third period, the differential between the electric charges integrated respectively in the second and third capacitors of the CDS circuit is determined by the differential calculating circuit, and a signal voltage corresponding to this differential is output by the differential calculating circuit. The signal voltage output by the differential calculating circuit corresponds to the spot light component. Either the first or the second periods may come prior to the other.
Moreover, the photo-detecting apparatus according to the present invention is characterized in that: (1) N sets (where Nxe2x89xa72) of the photodiode, the integrating circuit, the CDS circuit and the differential calculating circuit are provided; and (2) it further comprises: N hold circuits provided respectively with respect to the N differential calculating circuits, for holding and outputting the signal voltages output by the respective differential calculating circuits; and (3) an A/D converting circuit for successively inputting the signal voltages output respectively by the N hold circuits, converting the signal voltages to digital signals, and outputting the digital signals. In this case, N sets of a photodiode, integrating circuit, CDS circuit, differential calculating circuit and hold circuit are provided. A signal voltage output by the differential calculating circuit in each set is held by the hold circuit. Thereupon, the signal voltage output respectively by the N hold circuits are input successively to the A/D converting circuit, which converts these signal voltages to digital signals and then outputs the digital signals. In other words, a one-dimensional or two-dimensional image is captured, and this imagining result is output as a digital signal.
Moreover, the photo-detecting apparatus relating to the present invention is characterized in that it further comprises: a maximum value detecting circuit for detecting the maximum value of the signal voltages output respectively by the N differential calculating circuits or the N hold circuits; wherein the A/D converting circuit sets an A/D conversion range on the basis of the maximum value detected by the maximum value detecting circuit. In this case, the maximum value of the signal voltages output respectively by the N differential calculating circuits or hold circuits is detected by the maximum value detecting circuit. Thereupon, an A/D conversion range is set in the A/D converting circuit, on the basis of the maximum value detected by the maximum value detecting circuit.